The fence starts about eighty feet out into the Pacific. It's made of metal pylons and looks like a procession of old telephone poles, each jutting about twenty feet above the waves. The pylons are spaced tightly together, and there's a sign warning of additional barriers below the waterline. Once the fence hits dry land, it marches east across the beach, and then, on a little hill that begins where the beach ends, it changes. It becomes, in fact, two fences, a double barrier. Compared with the single-ply barrier on the beach and in the water, these two fences — sturdy square beams supporting tight rows of whitewashed steel spindles — look much more modern and formidable, like prison fences. One of the two fences picks up right where the beach fence leaves off and continues east along the actual borderline, while the other follows a parallel line a few dozen feet to the north of it.

The buffer zone between the two fences is reserved exclusively for the use of the U. S. Border Patrol, with one exception: At the top of the hill, there is a little door in the northern fence, and a sign informs that twice a week, Saturdays and Sundays from

10:00 A.M. until 2:00 P.M., U. S. citizens are allowed to enter. Then, if there happen to be Mexicans on the other side of the second, southern fence, the Americans are allowed to look at them and talk with them, though reaching through the fence or attempting "physical contact with individuals in Mexico" is prohibited. A portion of the American side of the visiting area has been paved with cement, in the shape of a semicircle, and there is an identical semicircle on the Mexican side of the fence.

A big marble obelisk stands in the center of the circle. There is a break in the southern fence to accommodate the obelisk, and some additional fencing around the break to keep anyone from trying to squeeze through.

In 1851, some men from something called the International Boundary Commission placed the obelisk here. Back then, the Mexican-American War had just ended, and Mexico had agreed to surrender more than half its territory to the United States, including the places now called California, Arizona, New Mexico, and Texas. The job of the International Boundary Commission was to come up with a map of the revised frontier between the two countries. They started here, on this beachfront hill, and installed the obelisk as their first survey marker.

Then they walked east, into the borderlands.

A geographer described accounts of the International Boundary Commission's expedition as the "stuff that dime novels are made of," complete with "deaths from starvation and yellow fever, struggles for survival in the desert, and the constant threat of violent attacks by Indians and filibusters."

Back then, of course, those surveyors had no choice when it came to transportation: In order to see the border, they had to travel either by foot or by horse.

Today there are lots of alternatives. You could fly from San Diego to the Gulf of Mexico in a few hours, or could drive the distance in a few days.

But there's still something to be said, when you want to really understand something, for slowing way down.

So this morning I'm taking my cue from the men who planted this obelisk.

Part 1: Imperial Beach, California, to Ajo, Arizona
This story is the first in a yearlong series chronicling Luke Dittrich's walk along the entire 1,933-mile U.S.-Mexican border, from Imperial Beach, California, to Brownsville, Texas. The first 350-mile section — illustrated on the accompanying map, which continues on the following pages — begins in the water of the Pacific Ocean near San Diego, then heads east over various mountain ranges before hitting the sprawling Sahara-like Yuha Desert. It ends with a 120-mile stretch along the Camino del Diablo, through a rugged and desolate landscape of lava rocks, sand, and cacti that was once traversed by the Papago Indians as a rite of manhood. Map by Bryan Christie.

It heads due east from the beach straight across California until it hits the Colorado River, at which point it backtracks a bit, squiggling southwest along the river's edge before firming up again and slicing across the bottom of Arizona in two long straight lines. Shortly after reaching New Mexico, it suddenly jogs north for a few dozen miles, but then quickly resumes its straight, eastward course all the way to Texas, where it merges with the Rio Grande and rides out the final stretch to the Gulf.

The border's complicated.

From a distance, it looks like an impossible tangle of Minutemen and La Migra, drugs and money, fence builders and fence hoppers. It's tempting to look away. But we shouldn't. The border is the place where we end and they begin, which makes it the definition of a defining place.

Planning a walk along the border, you quickly encounter certain problems.

One problem is political. The only feasible way to walk the actual borderline is to follow the dirt roads used by the Border Patrol, but a lot of those roads appear only on proprietary maps that the Border Patrol refuses to give out to members of the public. You can turn to online satellite imagery, but these days even that can't keep pace with how quickly new border roads are being plowed.

Another problem is geographical. The borderlands, whatever route you sketch through them, are a rough mix of deserts and mountains.

Sometimes problems meld the geographic and the political, because sometimes politics dictate geography. Example: I'm in an area known as Smuggler's Gulch, just east of the Friendship Circle. My maps show a deep ravine, one that drug and human traffickers used for decades to ferry their goods across the border. But a few years ago the Department of Homeland Security, armed with congressional permission to waive a number of environmental laws and regulations, sent in earthmovers to decapitate some nearby hills, filled the ravine with the resulting 1.7 million cubic yards of dirt, then topped it with a Border Patrol road and floodlights. Smuggler's Gulch, an ancient wrinkle in the earth, has been Botoxed. My maps are wrong.

My plan is to stick to the border roads where practical, where they exist, where I can find them. But I won't be too scrupulous about hewing to the line. Where there isn't a border road, or where the route looks more interesting a little inland, I'll let myself drift north. My first chunk is the 350 miles that now stand between me and Ajo, Arizona, including the 120 miles of open desert that immediately precedes Ajo, a stretch called El Camino del Diablo.

And that leads to the final problem: water. El Camino del Diablo, like a lot of places along the border, is dry. One hundred twenty miles' worth of water weighs a lot more than I can carry on my back. If this were a century and a half ago, if I were one of those obelisk-planting surveyors, I'd probably have opted to bring along a mule.

I look up, but I don't stop leaning. The stroller, if you add its own weight to the weight of all the gear and food and water inside it, weighs more than 120 pounds. The incline here, near the top of Otay Mountain, a dozen miles east of the beach, is steep, at least 45 degrees. If I stop leaning, the stroller will roll backward, over me, on down the slope.

The agent is standing on the top of the rise, looking down. He's holding a pair of binoculars.

"I saw you coming from a ways away," he says.

The border is approximately nineteen hundred miles long, and there are approximately eighteen thousand Border Patrol agents tasked with protecting it. That's nine agents per mile. Of course, these agents aren't posted at strict and regular intervals along the line. They move around, they cluster, and sometimes they pursue leads or man checkpoints up to a hundred miles from the frontier. But still. If you're walking the border, you're going to see a lot of Border Patrol.

I push the final few feet to the top of the rise and lock the stroller's wheels and stop to chat with the agent.

The eastern flank of Otay Mountain drops two thousand feet into a deep valley that runs north to Highway 94 and south to Mexico. I can see the fence, about a mile away, and some cars passing by on the other side of it. That's where they cross, the agent tells me. It's best to spot them as soon as they hop the fence, when they're exposed, because once they enter the thick foliage of the valley, they become a lot harder to see.

I give him back his binoculars and keep walking. Otay Mountain is the highest peak for miles around, and this particular spot has a great view of the ocean. The chaparral that clots the slope — the redshanks, the monkey flower, the mission manzanita, the sugar bush — fuzzes into a blue-green pastel as the slope descends toward the Pacific, which coruscates mildly in the distance. Maybe it's just the pollution, the haze of the San Diego — Tijuana megalopolis, but everything has a soft focus up here.

I hope to average twenty miles a day, but the mountain is steep and the cart is heavy and I only make it ten today before the sun drops away completely. I stop and make camp on a clearing beside the trail. I'm tired, and fall right asleep, then spend the rest of the night waking every couple of hours to the rumble and glare of patrols passing my tent.

YUHA DESERT The shoes of an illegal immigrant caught trying to cross the border in the Yuha Desert west of Calexico. He and others caught with him had glued foam rubber to the bottoms of their shoes to try to mask their tracks.

Coming down off the mountain is a lot easier than climbing up it, and I can relax a bit and let the rhythm of the walk begin to establish itself.

Every few miles, I'll run into an agent, who'll ask what I'm doing out here. Sometimes he'll ask to see the soles of my shoes. Agents spend most of their time cutting sign, which is to say, they patrol dirt roads near the border, looking for fresh footprints or other sign of aliens. When they come across people who are not aliens, they often ask to see the soles of their shoes. That way they won't later confuse native sign for alien sign.

Sometimes I'll see agents even when they're not really there. I'll spot their bright white-and-green vehicles parked on almost every significant overlook, but it's not till I'm right up on them, peering through the tinted windows, that I can tell whether they're occupied or just expensive scarecrows. About a third are empty.

The trail from Otay Mountain feeds into State Route 94, and I follow the highway east for about ten miles, then cut south toward the border again.

I spot a truck, and this one has an agent inside. I tap on the window and he rolls it down and gives me a nod. People call this town Tecatito on account of how it sits right across the border from the much bigger town of Tecate, Mexico. The agent's got the nose of his truck pointed straight south, where every so often someone walks out of the Customs building and into America. A poster pasted to a wall in his line of sight features head shots of ten Hispanic men, along with details of the crimes they're wanted for, mostly smuggling, some kidnapping, some murders. I tell him I'm going across, that my hotel's a couple of miles away, that I'll have to walk through most of Tecate to get there. Does he think I'll have any problems, safetywise? Tecate's not too bad these days, he says. From what he hears, anyway. He's never crossed himself.

The passport-control booth is empty. Nobody's there to look at my ID or ask to see what's inside the stroller, so I just walk across the line.

Let me make my prejudices clear: I love Mexico. I lived in this country for a couple of years when I was a kid, and I used to go back all the time. I love the language, the food, the pace, the people, the temperature.

The last time I visited, I was driving around a small city with an off-duty police detective, and the car we were in was his own, and he wasn't wearing a uniform, and he was just cruising at first, relaxed, a big tough guy spinning stories about some of the scrapes he'd been in, but I'll always remember the jolt that went through his body when at a stoplight he suddenly realized he'd left his wallet with the badge in it lying open on the dash, and how fast he scrambled to snatch it and hide it away, and the look on his face as he shot glances at the other vehicles stopped at the light to see if anybody had noticed.

That kind of fear is contagious.

And I hate it, how this fear works its way into my experience, how it becomes as tangible a part of the background texture of Tecate as the uncatalyzed exhaust or the swollen-titted dogs or the snakeskin boots or the sweet little old lady who gives me directions to my hotel and then says, "Dios te bendiga" as I'm walking away.

Because if you scrape away the fear, if you dig through it, or just look past it, all the best parts of Mexico are still here.

Tonight I eat at a restaurant near my hotel and they bring me flank steak and grilled nopale cactus and homemade corn tortillas and flan and a couple bottles of the local brew and a shot of Tres Generaciones and it is, without a doubt, the best meal I've had in months.

ALL-AMERICAN CANAL More than five hundred migrants have drowned in the All-American irrigation canal, which runs along eighty-two miles of the border. Last fall authorities spent $1.1 million to install 105 buoy lines and 1,400 warning signs.

Agent Muñoz shines the light through a scrim of trees, toward the tent.

"Anyone else back there?"

No.

We're a few hundred feet north of the border, on a low rise beside a dry creek bed, eighteen miles east of Tecate. It was a long walk today, pegged to the fence, lots of hills. The coastal flora had faded and hints of the high desert — tumbleweeds, the occasional cactus — had begun to show.

I tell him I chose it for the location, because it's not right next to the fence, and because the trees are thick enough that I can't even see the fence, which means that nobody at the fence can see me. I tell him I chose it because that's one thing everyone told me was very important: to avoid camping right next to the fence, or even in places where I could be seen from the fence. Because I'm most vulnerable at night. Because bandits on the other side could spot me, scramble over, take whatever they wanted, then scramble back, untouchable.

Everything I thought made this a good place, he explains, is exactly what makes it a bad place. This deep creek bed, shielded by its canopy of trees, is like Smuggler's Gulch before Smuggler's Gulch became Smuggler's Flat. The cartels love it, and drug mules come through all the time.

"We've got some pressure sensors buried farther up the creek," he says. "We send out a team anytime they get a hit. But that takes a while."

Should I pack up my tent? Would I be safer somewhere else?

He shakes his head.

"Nowhere's safe around here."

Before he gets back into his truck and drives away, he gives me the direct line to the nearest Border Patrol station, says I should call immediately if I see anyone.

"Tell them you're in the bottom of La Gloria. Everyone knows where that is."

The trees shimmer and wobble in the red glow of his receding taillights, and then it's dark again and I go and gather up everything I think I might be able to use as a weapon, including the pepper spray, a knife, and some hiking poles. I bring it all inside the tent, crawl into my bag, zip up, and lie there, waiting. Every so often, I hear something moving outside, crunching seedpods or snapping twigs, and I turn on my headlamp and scoot up and try to look out through the tent's wall of mosquito netting, but the netting catches the light, and all I see is the wall itself. Then I turn off the light until I hear something else. Lying there in the dark, watching vague shadows on the polyester, it feels like a world of unknowns is outside pressing in.

IMPERIAL SAND DUNES
Before 2008, the Imperial Sand Dunes were a hotbed of drug-trafficking activity, as smugglers would simply blend in with the usual crowd of helmeted ATV and dune-buggy drivers, hauling their loads across with relative ease. Now there's a fifteen-foot-high and roughly ten-mile-long "floating fence" that rides atop the dunes and has dramatically slowed crossing attempts.

Another gust lifts the dirt and sand off the road and spits it in my face, and I've had enough, so I take a break and lean sideways against the fence, my back to the wind. It's been blowing all day, fierce. The fence here, a few miles past La Gloria, is crude but strong, made from corrugated sheets of brown and yellow steel. The National Guard was deployed to build this stretch back in 2006, and every couple of miles a different engineering battalion marked the section it was responsible for by carving a piece of metal in the shape of its home state and welding it in place. The fence builders came from Hawaii, Wisconsin, South Carolina, and when they got here, they worked till they were done, even through the holidays, I'm guessing, judging by the spray-painted Christmas tree I'm leaning next to.

The fence is tall, twelve feet or so. It would be tough to climb over. And the steel of the fence would be almost impossible to tear through. But dirt is dirt, and from where I'm standing, I can see a spot where the dirt next to the fence has been dug up and there's a little hole underneath it, and I bet I could get down and crawl right through.

There's a sign on the other side of the road. It's bright yellow and busy with pictographs: A sun, some mountains, a rattlesnake, a cactus, and a little drowning man, one arm raised, sinking into a pool of water. CUIDADO! the sign reads. NO VALE LA PENA!

The returns are as stark and clear as those pictures on the sign. By the simple act of carrying his own body across the line, a man immediately boosts his earning potential sixfold. And if he chooses to carry something else along with his body, well, a pound of cocaine costs twice as much in Tecatito than it does in Tecate.

It's worth the trouble, and so every year roughly 2.5 million people in Mexico make the simple and consummately rational decision to make unauthorized entry into the United States of America.

It's worth the trouble, and so the fence, like the Border Patrol, has more than doubled in the last decade. But the incursions haven't stopped. Instead, they've shifted. Back near San Diego, where the fence is particularly brawny and the density of the Border Patrol is particularly high, traffic has slowed. Here, where the fence is easier to conquer and patrols are less frequent, traffic has increased. The pressure on the other side remains the same, and as long as there is any gap, any weakness, it will push and probe until it finds a way.

It's worth the trouble, and so I wonder why I still haven't run into any crossers myself. The wind eases a bit and I lean back into the stroller and start moving again. I've been hearing things all day, seeing things, too, little murmurs behind me or blurs of fabric in the chaparral or footfalls on the other side of the fence. I'll stop and look and listen, but then everything goes still and blank.

Eventually I come to a spot where the road and the fence split: The fence continues east over some particularly steep and rocky hills while the road jogs north. I follow the road until it crosses an abandoned train track, and then bump along over the slats for an hour or so until I reach Old Highway 80, which takes me back south. The highway brings me to a long concrete bridge, and there's a good view of the fence coming down out of the hills to the west, back onto the flats. At the end of the bridge, there's a big weathered green-and-white painted sign that says, wELCOME TO JACUMBA. A smaller sign hangs beneath the big one. It features a little drawing of a woman dressed in a 1910-style bathing suit and the words JACUMBA HOT SPRINGS ... ALL YOUR WANTS.

The manager's smoking a cigarette outside when I get there, and there are plenty of rooms available, and he gives me the key to one and I go and take a shower and change, and when I come back outside, the manager's still standing by the street, smoking another cigarette. He's a skinny guy, maybe forty, maybe fifty, with a tight-cinched belt and a sort of permanent smirk. There's a convenience store — Mountain Sage Market — across the street from the hotel, and it's open, and so's the Laundromat next to it, but most everything else here on the main drag — a car wash, a gas station, an antique shop — has gone out of business. In a vacant lot near the shuttered car wash, a clutch of Border Patrol agents are milling around, waiting for something.

I ask the manager about Jacume, which is the town directly south of Jacumba, right across the fence. I'd read about Jacume. The Los Angeles Times calls it a "black hole," says it's overrun by smugglers and that even the Mexican cops won't go near it. The manager tells me that Jacumba and Jacume used to be as close as their names imply, that before the fence went up, people from Jacume used to cross all the time to work day shifts and do their shopping here in Jacumba, and people in Jacumba used to cross all the time to eat or party in Jacume. Jacumba and Jacume, the way he tells it, used to be real border towns, meaning places where north and south sort of overlapped and mixed together. Now they're just towns on the border.

"I hear Jacume's real dangerous now," he says. "But I don't really have any idea what goes on down there."

A few minutes later, some sort of silent alarm must go off, because the agents in the vacant lot grab their M4's from the front seats of their vehicles and take off running toward the fence.

After Jacumba, the fence road fades out again, and I have to stick to the asphalt. I stay on it for a week, averaging twenty-five miles per day.

Highways are their own sort of wilderness. They were never meant for walking on, and their unrelenting sameness, their day after day of minutely incremental progress punches holes in your reservoir of memories, so that the narrative drains away and a whole week reduces to a dry heap of disconnected impressions.

The cherry-red semi that passes too close, too fast, and nearly pulls me into its wake.

The little leather pouch lying on the shoulder of I-8, the word COLUMBIA stenciled on it, and the mix of pesos and pennies inside.

The flat I camp on off of highway 98 in the Yuha Desert, and how long it takes me to find a clear spot for my tent, one without broken glass or bullet casings.

The days I take off in a hotel along the way, tending to my feet, hobbling back and forth to a Walmart pharmacy, trying Epsom salts, moleskin, Vaseline, duct tape, new socks, anything at all that might make the blisters hurt less.

The middle-aged guy in the hotel bar who proudly shows me his teeth, gleaming new crowns he bought across the border in Algodones for a third the price he would have paid in the States.

The bartender in the same hotel, who once spent a summer helping build the fence, telling a story about how the cartels cloned some of the vehicles and uniforms that belonged to one of the contracting companies, exact replicas, so they could drive their stuff freely back and forth, unsuspected, the whole time the fence was being built.

The Imperial Dunes, those odd chunks of Saharan desert on the eastern edge of California, buzzing all day and night with tricked-out hot-rod sand buggies.

The retired Border Patrol agent in Yuma who says he loved his job, then quotes Hemingway: "There is no hunting like the hunting of man."

The way the road rubs the same parts of my feet the same way, hour after hour, and how finally the blisters on my heels get bad enough that I start jogging instead of walking, just to change the point of impact.

The relief when I finally get off the highway in Wellton, Arizona, and the other emotion when I look south of Wellton, at the desert, the mountains, and the beginning of the Camino del Diablo.

YUMA
The border is divided into nine sectors. A recent federal report on border security declared that of the nine, Yuma was the only one to have achieved "operational control" of its section of the border.

In August of 1905, a wandering prospector named Pablo Valencia departed Wellton and headed south into the desert in pursuit of a lost gold mine. He was about forty years old, 155 pounds, rode a good horse, and carried along with him two two-gallon and two one-gallon canteens, for a total of six gallons of water, along with plenty of bread and sugar and cheese and coffee and tobacco and a sort of wheat meal called pinole. The first day he rode thirty-four miles and reached the spot near the southern tip of the Gila Mountains where the trail from Wellton intersected with the Camino del Diablo. Just to the west, a steep slope led up to a place known as Tinajas Altas, or High Tanks, where eroded stone basins usually contain pools of rainwater runoff. Tinajas Altas is the only semireliable water source along the Camino, and Valencia refilled his canteens there. As it happened, a self-taught geologist named WJ McGee had set up a camp nearby, working on a summerlong project to monitor the heat and humidity of the surrounding desert. The two men dined together on jerked mountain-sheep meat before Valencia saddled up again and rode east.

Eight days later, just as dawn broke, McGee heard an inhuman sound, like the roaring of a lion, near his camp, and followed it to its source. He later described what he found in a paper called "Desert Thirst as Disease," which ran in a 1906 issue of the Interstate Medical Journal.

Valencia, who just the week before had been "of remarkably fine and vigorous physique — indeed, one of the best built Mexicans known to me," was now "stark naked; his formerly full-muscled legs and arms were shrunken and scrawny; his ribs ridged out like those of a starveling horse; his habitually plethoric abdomen was drawn in almost against his vertebral column; his lips had disappeared as if amputated, leaving low edges of blackened tissue; his teeth and gums projected like those of a skinned animal, but the flesh was black and dry as a hank of jerky; his nose was withered and shrunken to half its length; the nostril-lining showing black; his eyes were set in a winkless stare, with surrounding skin so contracted as to expose the conjunctiva, itself black as the gums; his face was dark as a negro ... his lower legs and feet, with forearms and hands, were torn and scratched by contact with thorns and sharp rocks, yet even the freshest cuts were as so many scratches in dry leather, without trace of blood or serum; his joints and bones stood out like those of a wasted sickling, though the skin clung to them in a way suggesting shrunken rawhide used in repairing a broken wheel. From inspection and handling, I estimated his weight at 115 to 120 pounds ... The mucus membrane lining mouth and throat was shriveled, cracked, and blackened, and his tongue shrunken to a mere bunch of black integument."

In the long history of people running out of water on the Camino del Diablo, there are two things that make the case of Pablo Valencia unusual.

First is the fact that WJ McGee, such a meticulous observer, was there to chronicle it.

Second is the fact that Valencia survived and eventually recovered.

I walk south from Wellton for a day and a half before I reach Tinajas Altas, where I leave my stroller on the trail and scramble up the rocks to the lowest tank. It's half full of greenish, algae-topped rainwater. A quarter mile away lies the "mesita de los muertos," a little mesa topped with anonymous wood and stone crosses under which lie the bones of sixty or so — no one knows the exact number — men, women, and children. Some of their bodies had been discovered within sight of these tanks, some right at the base of this rocky slope, poor souls too weak to pull their own desperate husks up to the water above.

I head east from Tinajas Altas, onto the Camino del Diablo, with eight gallons in my stroller.

The geography, the remoteness, and the challenges of the Camino have remained more or less constant since Pablo Valencia's time, though there is one new hazard he would have found bewildering: A big chunk of it runs through the U. S. Air Force's Barry M. Goldwater Range. To gain entry, I signed a liability release that read, in part, that I accepted the "danger of property damage and permanent, painful, disabling, and disfiguring injury or death due to high explosive detonations from falling objects such as aircraft, aerial targets, live ammunition, missiles, bombs, etc."

The Tinajas Altas are less than fifteen minutes behind me when I hear a huge roaring sound. I look to the north and see, barreling toward me close above the desert floor, two F-16 fighter jets. Before they reach me, they pull up and shoot nearly vertical, chasing each other into the blue sky.

THE ROAD INTO EL CAMINO DEL DIABLO Just outside Wellton. From here it is 120 miles through the desert with no guaranteed source of water.

It's early afternoon, my third day on the Camino, and the sand on this part of the trail is too deep for my stroller, so I'm up on the thin crust of the surrounding desert instead, navigating around the cholla spikes and the ironwood stumps, my feet or tires or both occasionally dropping down into a rabbit hole, when I hear something behind me and I turn and there's a Border Patrol truck coming, kicking up dust. I stop and watch it approach.

Two agents. Both Hispanic, early thirties. Neither is wearing a uniform. The passenger gives a curt nod in my direction, but the driver offers a big smile, asks what I'm doing out here, and where I'm going.

I tell him.

"Ajo!" he says. "That's a long way away."

They drive off and I keep walking, and pretty soon I've fallen back into my stupor, not thinking anything at all, until about a half hour later I notice that, way ahead in the distance, the same Border Patrol vehicle is stopped at a bend in the trail, just parked there, idling.

Your head does funny things.

You're all alone for hours at a stretch, except for the fighter jets and the occasional long-eared Sonoran rabbit, and you might think the solitude and the Biblical terrain would lend itself to deep contemplation, but it doesn't, not really. You mostly find yourself falling into half-trances of no thought at all, just the constant plod, until some unexpected stimulus intrudes and seizes hold of your mind.

I start thinking about the guys in the truck. Why weren't they wearing uniforms? And why was the passenger so curt, so unfriendly? And why, for that matter, was the driver so friendly? Then I start thinking about the cartels and how good they are at cloning vehicles.

I move a little ways farther off the trail, though it seems a pretty futile gesture. People say things like "There's nowhere to run" all the time, but right now, besides me and these two guys there might not be another human being for forty miles in any direction.

When I'm even with the truck, but about a hundred feet off the trail, I look over and the driver is still sitting in the driver's seat, and he gives me another smile and a wave. I don't see the passenger for a moment, and then I notice that he's lying next to the truck, partly beneath it, looking at something on the undercarriage, and there's a little toolbox next to him.

More hours, more miles. When the sun starts going down, I'm in a Martian landscape of jagged orange-black rocks, the Pinacate lava field. It's cold, below freezing, a lot colder than it's ever supposed to be in Arizona. By the time I've got the tent set up, my blood has slowed and the air is numbing my fingers. I eat quickly and then get into my bag wearing everything but my shoes, a wool hat pulled down over my ears.

I sleep for a while, until the wind wakes me.

The wind is amazing. This is a three-person tent, and I'm lying in the middle of it, and there would usually be plenty of floor space to my right, but now the wind has lifted up that whole side, ripping out all the stakes, and is blowing so hard that the floor has doubled over like a taco shell, coming down

on top of me. It's a wonder the tent poles haven't snapped. I unzip my bag enough to get my hands out and push the floor up off of me and the wind whips the polyester around my hands, and if someone were outside watching the tent right now, I bet I would probably look a little like Han Solo stuck in the carbonite. Eventually the gusts die down and the floor settles and I zip all the way up again and try to sleep, but then I realize I need to piss.

It's 1:00 A.M. and the rocks don't look orange-black now. In the moonlight they just look gray, like everything else. I notice that a water bottle I left outside is iced up. The wind starts up again while I'm pissing, and I have to hold on to the tent with my other hand to keep it from tumbling off across the lava flow.

There's a red light throbbing in the distance. It's maybe a couple of miles to the north, just above the top of a mountain, and at first I think maybe it's a radio tower or something, but then I look at it longer and it doesn't look like a radio tower. It looks like it's floating. Like it's some sort of orb floating there above the mountain, stationary, throbbing, red. Another night I saw a different mysterious light, but that light was on the ground, and it looked like a flashlight beam, and I thought it was coming my way, and it scared me — because who would be out here and why would they approach my tent at night? — and so I had grabbed my satellite communicator and held it with my thumb poised over the SOS button and watched the beam until it disappeared behind some rocks in the distance. But this light tonight, this throbbing, floating, luminous orb, what the hell is it? It's not scary. Just intriguing. My camera's in the tent. I should get it, take a picture. But by the time I crawl back inside, I'm so cold that the idea of going back out into the wind, even to photograph a UFO, is just too much.

EL CAMINO DEL DIABLO
In the past thousand years, the Camino del Diablo has been walked on by Papago tribespeople, Gold Rush strivers, and Edward Abbey, though who first created the trail is unknown. It is a mix of desert scenery, cholla beds, sand dunes, mountains, and lava fields.

The Camino del Diablo is older than America.

It extends from Tinajas Altas for a hundred miles, straight through the lava fields and sand dunes and cholla beds of the Sonoran Desert. Nobody knows who created the trail, but a thousand years ago it was already ancient, and young Papago Indians would make holy pilgrimages on it, testing their manhood. A regiment of conquistadores in pursuit of El Dorado may have ridden here in the 1500s, but the first Europeans who definitely set foot on the Camino were Jesuit missionaries in 1699, on the expedition that established the first overland route to the Spanish colonies on the West Coast.

It earned its name during the Gold Rush, in the 1850s. Hundreds of ill-prepared miners and their families dried up and died here, trying to reach the gold fields of California. The arrival of the transcontinental railroad in the late 1870s made the route obsolete, and hardly anyone else set foot in the wilderness around the Camino until recently, when other groups of strivers began to pass this way.

The Camino runs parallel and close to the border for most of its distance, and from the Camino north to I-8 there is an average of about forty miles of open desert. As border security has increased around the more densely populated zones to the west, the Camino area, despite its challenges, has become a popular thoroughfare for migrants and smugglers. On a Saturday in May 2001, for example, fourteen men — a mix of former coffee farmers, citrus-plantation workers, Coca-Cola plant employees, and high school students — were dropped off near here, near where Mexico's Federal Highway 2 skirts the very edge of the border, and told that if they walked north, across the Camino del Diablo, across this desert, they would reach an American highway. And so they walked north. That evening they came up against the Growler Mountains, which they tried and failed to cross. They walked north along the edge of the range for most of the next day, and then for some reason they turned almost 180 degrees and walked southwest across the valley that divides the Growler Mountains from the Granite Mountains, walking all the way to the southern tip of the Granite range, back near the border, before turning again and heading northwest.

If you look at their route on a map, if you trace their increasingly lost and desperate wanderings, which lasted a total of five days, it looks like the stem of a weak seedling, the way a weak seedling rises up, doubles back down toward the earth, and then tries to rise again, reaching hungrily toward the sun.

Of course, they had more than enough sun.

And not enough water.

And when Border Patrol agents found them, there was little they could do but collect their remains and their belongings — the blue jeans, the belt buckles, the white shoes, the combs, the fake silver watches — and turn these things over to the Mexican Consulate.

They had individual names — Julian, Arnulfo, Reyno, Claudio, Mario, Lauro, Enrique, Reymundo Sr., Reymundo Jr., Abraham, Edgar, Efrain, Lorenzo, Heriberto — but there were so many of them that it was much easier simply to refer to them as a collective. The Yuma Fourteen. None ever made it to Yuma, but Yuma is the nearest big city.

As in the case of Pablo Valencia a century before, there was nothing unique about how the Yuma Fourteen ran out of water, or what happened to them afterward. More than a hundred people had already dried to death that year in this same desert.

But the sheer number of deaths, all at once, was impressive, and there was pressure to do something about it, to respond in some way.

Camp Grip, which was built in 2002, is one of those responses. The "Grip" stands for the forward operating base's mandate, which is to "get a grip" on this stretch of the border.

I see Camp Grip's radio tower come into view first, then the trailers and the trucks. The trailers are new and clean and there's a barbecue going outside one of them and when I get close, I can smell hamburgers cooking and it smells good. I ring the doorbell. I wait a minute and there's no response, so I ring the doorbell again and knock, too. The air-conditioning unit on the side of the trailer is on, and it's loud.

We chat a bit. Camp Grip has been a success, in the sense that traffic is way down from the worst times, he says. But there's still plenty of traffic. I tell him about the red light I saw last night above the lava field. He shrugs.

"There was a big group crossed last night near where you camped," he says. "We caught them earlier today, about twenty miles north. They could have had someone up in the mountains, scouting for them. That light might have been their go-ahead signal."

He walks to the barbecue and opens it up and grabs a spatula hanging from the side and flips the burgers.

THE ROAD OUT OF EL CAMINO DEL DIABLO
Dittrich and his stroller on day 22, the last of his initial 350-mile trek. The stroller's wheels are reinforced with internal polyurethane strips. Packed, it weighs more than 120 pounds and contains eight gallons of water and gear including a tent, sleeping bag, camping stove, satellite phone, and Spot satellite device, which automatically transmits his position to a Web site where friends can track him.

The rising sun lights up the inside of the tent and makes everything glow orange, and I wait until the air warms a little and then I sit up and unzip my bag and look at my feet. Every morning, this is the first thing I do. The cold of the night keeps them pretty numb, so I can't tell until I see them whether they've gotten worse or stayed the same.

The worst blister is on the outer edge of the left half of my left heel. It looks sort of like someone cut a Ping-Pong ball in half and stuck it there, though not quite as round or symmetrical, and the fluid inside is not milky white like a Ping-Pong ball but is instead pinkish with streaks of brown. I pull out my medical kit and dig around for alcohol swabs and wipe off the blister and the tight red skin around it. I take some Handi-Wipes and lay them down on the floor of the tent and rest my foot on the wipes, then dig around in the bag again until I find the safety pin. Most of it spurts onto the Handi-Wipes as soon as I punch the holes, but I squeeze to make sure it's all out. I wipe it clean with alcohol again, wait a minute for it to dry, then smear on some Neosporin and cover it all up with a new bandage. Once I'm satisfied with my work on this one, I turn to the others.

Every day starts like this, and every day once I'm done, once I've pulled on my socks and squeezed back into my shoes, I try not to think about my feet at all until the next morning.

The trail arcs gently north from my campsite.

I get on it and start walking the final sixteen miles to Ajo.

The New Cornelia Mine is an open pit a mile wide and a thousand feet deep, with an emerald pool of coppery rainwater at the bottom. I know that Ajo is just beyond the mine, but from the ground, approaching from the south, all I can see is a towering tailings dam, the junk rock left over from the ore-extraction process. It's the most massive dam of any sort in the country, 7.4 billion cubic feet of rubble, and it fills the entire horizon. For seventy years the people who lived in Ajo lived off the mine. They took from it what they could and piled the rest up high. Then copper prices crashed and in 1984 the mine shut down.

The trail has turned into a road, and I follow the road to the dam's leading edge, then around the dam to the northeast, until I reach the intersection with State Route 85.

I get on the shoulder and walk north. I'm looking forward to stopping for a while, to seeing my daughter, to letting my feet heal. I walk faster. Soon I'm on the outskirts of Ajo, passing by faux-Spanish-colonial buildings constructed during the boom years. They look a lot older than they are. When the men from the International Boundary Commission passed through this area a century and a half ago, mapping the new frontier, Ajo didn't even exist.

Behind me to the south it's a straight shot to the line, and there's not much between here and there except for the Ajo Border Patrol Station. The station opened on a patch of desert scrubland in 1987, when Ajo was just a backwoods old mining town with few prospects, and our biggest fears about Mexico concerned unfair trade agreements. The station looks a little run-down, a little overstuffed, too many cars crowded around too few buildings.

They've started construction on a new station, right next to the old one. Contractors from Tucson, soldiers from the Army Corps of Engineers, they're all down there now, clearing and grading the land, getting it ready. The existing station was designed for 25 agents. The new one will accommodate 360, with room for future expansion. When it's finished, next summer, it'll become Ajo's economic center, like the New Cornelia Mine once was. Soon one in four people living in Ajo will either be a Border Patrol agent, the spouse of a Border Patrol agent, or the child of a Border Patrol agent.

Twenty-seven miles farther south, the highway ends at the Lukeville crossing, and another new procession of steel and wire, built in 2008, marches east across the desert.